Most modern high temperature Ni base superalloy articles used in gas turbine engines, particularly those which are air cooled and have relatively complex inner cooling chambers and passages, are difficult and expensive to manufacture. Therefore, when such an article is damaged during engine operation, it is far more desirable to repair rather than replace the article. As a result, a variety of repair methods have been developed and reported.
Some reported methods use a form of fluoride ions to remove contaminants disposed on a surface in preparation for subsequent repair. For example, in preparation for the brazing type repair of a crack or crevice in an article surface, Keller et al in U.S. Pat. No. 4,098,450 (patented Jul. 4, 1978), remove oxides of Al and/or Ti by exposing the surface of such damaged portion to fluoride ions. Then a repair alloy is flowed into the cleaned portion. Modification of such use of fluoride ions is reported in U.S. Pat. Nos. 4,188,237 and 4,405,379 --Chasteen (patented Feb. 12, 1980 and Sep. 20, 1983, respectively), wherein it is intended to render article surfaces more brazable by cleaning through use of an organic resin as a fluoride ion source. Gases including fluorides have been used to decarburize surfaces as well as to act as a "getter" atmosphere for oxygen to attempt to avoid oxidation in some types of heat treatments.
All of such reported preparations of a surface for brazing do not address the problems associated with preparing a damaged portion beneath a surface of an article for welding in which a damaged portion, for example a crack, crevice, worn or abraded surface, or entire wall portion, is too extensive for brazing. In such an instance, a significant amount of repair alloy must be added by welding; and weld integrity, including weldabilty and adhesion, becomes a significant problem with engine-run Ni base superalloy articles, particularly in the repair of relatively thin outer walls of articles. The more highly alloyed advanced Ni base superalloys of the type manufactured as substantially single crystals or as directionally solidified elongated multigrain articles are particularly difficult to repair weld successfully. When the repair weld is at bands, projections such as platforms, and thin outer wall portions at which complex shapes exist, such as at the tips and leading and trailing edges of blades, the problem of repair welding becomes still more difficult.